The present invention relates to methods of fracture acidizing underground well formations, and more particularly to a new method of fracture acidizing whereby flow channels are etched to increase the production of oil, gas, and the like from the well formation.
Many methods have been developed and used successfully for increasing or stimulating the production of oil, gas, and the like from well formations. One commonly used method of well stimulation, where the producing formation is formed of acid soluble rock of relatively low permeability, is known in the art as fracture acidizing. The method consists of exerting fluid pressure on the formation to create one or more fractures therein, and then extending the created fracture outwardly from the well bore, often as far as possible. Either during the creation of the fracture, or after the fracture has been created, the faces of the fracture are exposed to acid. The acid may be pumped continuously or in stages, alternating with non-acidic, usually viscous liquid. High conductivity passageways for fluid flow to the well are created by the non-uniform acid reaction with the fracture faces, due to heterogeneous rock composition or fingering of the acid through the alternating stages of non-acidic, viscous liquid. The action of the acid on the faces of the fracture is often called etching. Non-uniform acid reaction with the faces of the fracture produces differential etching. In some treatment techniques, the fracture is allowed to close between multiple acid-treatment cycles. Once the desired acid volume has been pumped, or the desired contact time has been achieved, the well is returned to service.
Applicant is not aware of any prior art which, in his judgment as one skilled in the fracture acidizing art, would anticipate or render obvious the fracture acidizing process of the present invention. The following patents are described for the purpose of fully developing the background of the invention. U.S. Pat. No. 3,768,564 discloses a process known as Closed Fracture Acidizing, wherein fractures are allowed to close prior to prolonged contact with the acid. Flow channels are etched while the fracture is held open, then expanded only after the fracture is allowed to close. This process is also described in "Stimulating Carbonate Formations Using a Closed Fracture Acidizing Technique" by S. E. Fredrickson, a paper prepared for the Society of Petroleum Engineers East Texas Regional Meeting, held in Tyler, Tex., Apr. 21-22, 1986 (SPE 14654). U.S. Pat. No. 3,842,911 describes the use of propping agents in the Closed Fracture Acidizing process. The formation is acidized after the fracture closes on the propping agents, allowing creation of flow channels a relatively long distance from the well bore. U.S. Pat. No. 3,885,630 discloses the use of a weak acid that reacts slowly to etch flow channels in the reservoir. The method is particularly applicable to high temperature reservoirs, where strong acids may react too quickly. U.S. Pat. No. 3,918,524 discloses an improved method of fracture acidizing wherein the treating fluid is prevented from underriding preflush or other fluids in the formation and is caused to contact the faces of the fractures over their entire heights and lengths. U.S. Pat. No. 4,245,702 describes a process of fracturing and acidizing a well with the use of propping agents that is particularly applicable to relatively hard formations. U.S. Pat. No. Res. 30,484 describes an improved method of fracture acidizing wherein the treatment is confined to a desired zone within the formation. U.K. Patent No. 2,163,790 describes a gelled acid composition and a process for using the gelled acid in a fracture acidizing process.
It is known in the art that to achieve maximum effectiveness of the fracture acidizing process, it is often desirable to maximize the time the fracture is exposed to the acid, while limiting the amount of acid used to an economically reasonable amount. However, in fracture acidizing procedures used heretofore, less than desired results have often been achieved when the acid exposure time is maximized. For example, where the fracture acidizing treatment of a well formation has been carried out by first creating a fracture in the formation and then continuing to inject acid into the fracture at a high rate and pressure, in one or several stages, the fracture faces adjacent to the well are exposed to the etching of a lot of acid for a relatively long period of time,, and yet the fracture faces furtherest from the well may have received insufficient acid contact, even after a large quantity of acid has been injected. In some formations, the longer the acid is allowed to etch the rock faces adjacent to the well, the more likely that those rock faces will become softened or overetched, so that upon closing, the faces will crush against each other, effectively destroying or restricting the flow channels created adjacent to the well. In other formations, which react more slowly, the acid contact time and effective acid penetration into the fracture may be insufficient to provide additional flow channels at a distance not adjacent to the well.
By the present invention, a new method of fracture acidizing a well formation is provided, wherein acid or other fracturing fluid is first injected into the formation at a high rate, sufficient to create a fracture of the desired dimensions, and then acid is injected at rates that are relatively lower, but still sufficient to maintain a pressure on the fracture that will hold the fracture open without significant further extension. The injection rate after fracturing is approximately in equilibrium with the rate at which fluids leak off from the fracture. The leak-off rate declines over time, and acid may be injected at low rates for a relatively long period of time. Consequently, it is possible to achieve a longer contact time while injecting a smaller total acid volume for a fracture of a given size, relative to conventional fracture acidizing techniques.